ZS20 plus 2x extender

Ian and others have already shown that you can get useful results with stacked converters producing 2.55x or more so going to 4x is not unreasonable. In real terms it calls for a focal length of 344mm and an aperture of f/6.4.

If you have a subject that calls for a focal length equivalent to 2000mm in full frame terms then your choice is to buy a real 2000mm f/6.4 lens, which would certainly be very large and very expensive or the 4x extender, which would be expensive but nonetheless much more "affordable".

To be fair, there are other cameras that have much wider zoom ranges than the ZS20 and some of them could reach the 2000mm objective with existing extenders. However, most of us chose the ZS20 because of its small size and we don't want to lose that advantage so for us a 4x extender would make sense if it were available.